The present invention provides an improved pulse time-of-arrival detection apparatus for use with precision distance measuring systems. More particularly, this invention relates to a fixed-threshold pulse time-of-arrival detector which is adaptively responsive to incoming pulse waveforms in a manner which enhances the detector's ability to minimize multipath induced errors.
Distance Measuring Equipment ("DME") is associated with enroute and terminal area navigation, as well as nonprecision navigational techniques. Precision DME systems are designated "DME/P," while the nonprecision DME systems are referred to as "DME/N." One application for DME/P is as an integral element of the Microwave Landing System for aircraft Within the Microwave Landing System, DME/P performs a precision ranging function which complements the azimuth and elevation guidance functions of the Microwave Landing System.
Presently, two accuracy standards exist for DME/P systems; a 100 foot standard ("Accuracy Standard 1"), and a 40 foot standard ("Accuracy Standard 2"). To qualify under either of these standards, a DME/P system must be able to provide range information to a distant DME interrogator, within the specified tolerance, while overcoming errors induced by specular multipath, garble interference, diffuse multipath receiver noise, and instrumentation deficiencies. Today, only systems meeting the Accuracy Standard 1 requirements have been implemented and flight tested. However, the development of a DME/P system capable of achieving Accuracy Standard 2 is crucial, as such a system would allow a Microwave Landing System to expand its operational benefits to include Category II and Category III Computed Centerline Approaches (Category III is a weather condition defined by the International Civil Aviation Organization ("ICAO") having a ceiling of zero and very limited visibility, and Category II is a weather condition having a ceiling of approximately 100 feet with improved visibility). An Accuracy Standard 2/Microwave Landing System would be of particular value in applications involving short runways or runways with large DME/P offsets. Most importantly, Accuracy Standard 2 is necessary to permit a Microwave Landing System to direct an Area Navigation (RNAV) Short Turn-On-To-Final Approach landing. In such a landing, the straight-in portion of the Final Appproach is approximately one nautical mile in length. Systems meeting Accuracy Standard 1, regardless of the landing minimum, cannot satisfy the Path Following Error (PFE) or the Control Motion Noise (CMN) requirements for directing such a landing.
The primary source of error in DME/P systems is specular multipath, and the element most critical within a DME/P system for reducing specular multipath error (and therefore achieving Accuracy Standard 1 tolerances) is the pulse time-of-arrival ("TOA") detector. Prior art TOA systems have employed TOA detectors which use a Delay-Attenuate-Compare ("DAC") technique to provide an estimate of pulse arrival time. TOA detector multipath performance is evaluated as function of error vs. multipath delay, and, as shown in FIG. 1, a DAC TOA detector has an error of approximately 25 feet for a 100 ns multipath delay time. When this TOA detector error is combined with errors from other sources within the Microwave Landing System, the total system error exceeds 40 feet.
DME/P accuracy is achieved by thresholding low on the pulse leading edge at a time before the multipath arrives, and making the pulse arrival time processor independent of pulse amplitude and pulse shape variations. Because of its simplicity, a natural candidate for such a TOA detector is one employing a simple fixed threshold technique. However, the pulse arrival time processor for such a detector is not independent of pulse amplitude and pulse shape variations. This is true whether an automatic gain control or log receiver is applied.
For example, assume a linear receiver wherein threshold is defined by: ##EQU1## where t.sub.RT is the 10% to 90% pulse rise time, V is the peak pulse amplitude, and TOA.sub.M is the time that the pulse leading edge crosses the threshold. Further assume that the fixed threshold is within the linear region defined by the DME/P partial rise time specification of 5% to 30%. Let threshold=0.1 V and assume that tm=1000 ns. (ICAO requires that the pulse waveforms have rise times between 800 ns and 1200 ns.)
The virtual origin (or the start of the pulse) t.sub.vo, is obtained by solving the above equation for t=t.sub.vo, which yields: ##EQU2##
Taking the differential of equation (2), the pulse arrival time error due to pulse shape and pulse amplitude is: ##EQU3## Thus, a rise time error .DELTA.t.sub.RT of 200 ns, and an amplitude error .DELTA.V of -0.1 V, can result in a 30 ns TOA estimation error. Clearly, such a linear receiver would not be practical for use in a low error Microwave Landing System as a sizable portion of the Accuracy Standard 1 error budget is consumed by only the small amplitude and wave shape variations evident in the incoming pulse.
It is desirable then to employ a pulse arrival technique which is invariant to pulse amplitude and shape variation. Such an implementation is the DAC technique is described in Kelly, R. J., "System Considerations for the new DME/P International Standard," IEEE Trans. Aerospace, Electronic System AES-20, Nov. 1, 1984. A DAC technique system, using a 100 ns and -6 dB attenuation, is the industry standard. Such systems have been simulated, built and successfully flight tested, and have achieved Accuracy Standard 1. However, the DAC technique does not provide the precision necessary to satisfy Accuracy Standard 2.
Accordingly, it is the object of the present invention to provide for a TOA estimator employing a method of detection which reduces multipath error and enables Microwave Landing Systems to achieve an overall accuracy which will satisfy Accuracy Standard 2. This accomplished by employing an Adaptive Fixed Threshold ("AFT") technique TOA estimator.